1. Field of the Invention
The present invention relates to a method and system for time synchronization in a Broadband Wireless Access (BWA) communication system, and more particularly to a method and system for time synchronization between Base Stations (BSs) in a BWA communication system.
2. Description of the Related Art
The existing communication networks for providing common mobile communication services have been developed mainly in order to provide voice service and thus can transmit a relatively small quantity of data. Therefore, in comparison with other types of wireless communication systems, the mobile communication systems for providing voice communication services, as described above, have a relatively smaller bandwidth. Nowadays, the necessity for a communication system capable of efficiently providing Internet service is increasing due to users' increasing demand for the Internet service together with developments in the communication industry. However, the communication networks, developed mainly in order to provide the voice service, require relatively expensive charges for services requiring a large quantity of data, such as Internet service, because the communication networks mainly for the voice service have a relatively smaller transmission bandwidth.
In this regard, Institute of Electrical and Electronics Engineers (IEEE) 802.16 standardization group, which is an international standardization organization, is now driving to establish an IEEE 802.16d standard for providing a broadband wireless Internet service to a stationary subscriber station. A BWA system according to the IEEE 802.16d standard has a wider bandwidth for data transmission and thus can transmits a much larger quantity of data in short time, in comparison with the conventional wireless technology for voice service. Further, in the BWA system according to the IEEE 802.16d standard, all users can share a channel, thereby achieving efficient use of the channel.
In the BWA system, all users connected to a BS share a common channel and the BS allocates intervals of the common channel for use of each user at every uplink and downlink frame. Therefore, the BS must inform uplink and downlink access information to the users every frame in order to enable the users to share the channel. In the BWA system, access information includes uplink access information and downlink access information, and a MAP message containing the uplink access information or downlink access information is inserted to a front portion of each frame transmitted to the users. The MAP message includes various types of MAP information elements.
Meanwhile, the efficiency in using the frequency band in the BS is based on the frequency reuse rate. Therefore, it is necessary to make the frequency reuse ratio approach ‘1’ in order to increase the efficiency of using the frequency band. In order make the frequency reuse ratio approach ‘1’, it is necessary to achieve exact time synchronization between the BSs. Usually, a separate apparatus for time synchronization between all BSs is used in order to achieve exact time synchronization between the BSs. For example, each BS may be equipped with a Global Positioning System (GPS) receiver for receiving a GPS signal in order to achieve the time synchronization between the BSs, that is, in order to acquire time synchronization. Further, a BS equipped without such a GPS receiver can acquire time synchronization from another BS which has already acquired time synchronization.
However, when a BS without a GPS receiver performs time synchronization with another BS which has already acquired time synchronization with or without a GPS receiver, there has been no disclosed method for confirming if the time synchronization information transmitted from the another BS is available. A GPS signal transmitted from a GPS transmitter includes time synchronization information. A BS equipped with a GPS receiver acquires time synchronization directly from time synchronization information included in the GPS signal and then transmits time synchronization information corresponding to the acquired time synchronization to other BSs requiring acquisition of time synchronization. If another BS can use the transmitted time synchronization information within an available time range in order acquire time synchronization, it can be then said that the transmitted time synchronization information is available. Time synchronization information received by other BSs after passage of the available time range is not available.
Therefore, no problem with availability exists in a BS equipped with a GPS receiver and receiving a GPS signal directly from the GPS transmitter, because the BS can perform exact time synchronization by using the time synchronization information included in the directly received GPS signal. However, when a BS without a GPS receiver performs time synchronization with another BS having already acquired time synchronization without a GPS receiver, the BS performs the time synchronization without confirming the availability of the received time synchronization information. Therefore, it is impossible to recognize if the time synchronization has been performed precisely. That is, it is impossible to recognize if the time synchronization has been acquired based on exact time synchronization information provided by a BS having a certified availability. As such, it has been impossible to guarantee reliability in the acquisition of time synchronization between BSs.